EP 0834992 A2 discloses a monolithic MOS switched capacitor circuit with an on-chip oscillator.
U.S. Pat. No. 6,028,488 discloses a digitally controlled oscillator with switched capacitor frequency selection.
The simplest method for generating a cyclic signal involves counteracting the damping of an LC resonant circuit using an amplifier. The frequency stability of LC oscillators is not adequate for many applications. For high frequencies, L and C can be integrated. Much better frequency stability can be achieved with quartz oscillators. However, an external component is always necessary with such crystal oscillators.
Another group of oscillators are relaxation oscillators, which generate a cyclic signal by charging and discharging an (integrated/external) capacitance. In that case, the oscillator frequency is essentially dependent upon:                supply-voltage and temperature dependent current source/sink        switches in series with frequency-determining resistors;        a comparator with supply-voltage and temperature dependent hysteresis;        two comparators whose input stages differ by virtue of the different trigger thresholds and are therefore not matched to one another (mismatch).        